To date, the only licensed combination vaccine containing Haemophilus influenzae type b (Hib) conjugate vaccine and diphtheria and tetanus toxoid and acellular pertussis (DTaP) vaccine is for use in toddlers 15-18 months of age. The FDA and the National Immunization Program have become aware that in certain clinical settings, licensed Hib conjugate vaccines and DTaP vaccines are being combined for administration as a single injection in infants 2,4, and 6 months of age. These DTaP/Hib vaccines have not been licensed for combination use in the primary immunization series in infants. Clinical studies in infants conducted under Investigational New Drug Applications (INDs) have demonstrated that using some combination products may induce a suboptimal immune response to the Hib component. Therefore, only products licensed and labeled for use in the primary immunization series should be used in infants. In the event that an infant has received a primary immunization series with a DTaP/Hib combination, please contact the Immunization Hotline at the Centers for Disease Control and Prevention ( 232-2522 for English and  232-0233 for Spanish) as to further immunization actions that may be required.
Nightingale SL. Off-Label Use of Haemophilus Vaccine in Combination With DTaP Vaccine. JAMA. 1998;280(13):1128. doi:10.1001/jama.280.13.1128-JFD80009-2-1